A technique for avoiding magnetization saturation of a transformer by exciting current of the transformer is conventionally known.
FIG. 11 shows a power converter 8 comprising a transformer 81, a switch circuit 82 connected to a primary winding 811 of the transformer 81, a rectification circuit 83 connected to a secondary winding 812, and a smoothing circuit 84 provided in an output side of the rectification circuit 83. In FIG. 11, the switch circuit 82 is made up of a switching element Q1, and the rectification circuit 83 is made up of two switching elements Q21 and Q22. Moreover, the smoothing circuit 84 is made up of an inductor LO and a capacitor CO.
In the power converter 8, the transformer 81, which has a small excitation inductance (exciting current Iex is large), is used to enable flux reset of the transformer, and a current sensor CT is provided to the primary winding 811. Primary load current I1Load of the transformer 81 is reflected as the secondary current I2. That is, I2=n×I1Load, where a winding ratio is expressed by n (the number of the primary winding N1/the number of the secondary winding N2), so that the secondary current I2 can be measured by detecting the primary current I1, without directly detecting the secondary current I2. The power converter 8 is suitable for avoiding malfunction due to a noise of the secondary current, or reducing measurement loss (power loss at time of measurement), where the winding ratio n is large. FIG. 12 shows the primary current primary load current I1. I1road, the secondary current I2 and the exciting current Iex, and also shows relation of ON/OFF states of the switches Q1, Q21, and Q22 therewith.
On the other hand, a technology, which uses exciting current of a transformer in order to operate a circuit and apparatus provided in the primary side of the transformer, is also known.
FIG. 13 shows a power converter 9, in which a switch circuit 92 provided in a primary side of a transformer 91, is made up of a semiconductor switch, and power conversion is performed by a ZVS (zero bolt switching) method (refer to patent document 1). In this transformer 91, the excitation inductance of the primary winding 911 is intentionally made small.
In FIG. 13, the switch circuit 92 is formed by a bridge made up of switches Q11, Q12, Q13, and Q14 (MOSFETs in the figure). Direct current voltage DCIN is given to an input terminal of the switch circuit 92, and the primary winding 911 is connected to an output terminal thereof through a resonance circuit 95, which is made up of an inductor L1 and a capacitor C1.
Moreover, a rectification circuit 93, which is made up of diodes D21, D22, D23, and D24, is provided in a secondary side of the transformer 91. An input side of the rectification circuit 93 is connected to a secondary winding 912, and a load is connected to an output side thereof through a smoothing circuit 94 (capacitor C2).
A parasitism diode and a parasitism capacitance (capacitor) are formed in the switches Q11, Q12, Q13, and Q14, and for example, if the switches Q11 and Q14 are turned off while the switches Q11 and Q14 are turned on and the switches Q12 and Q13 are turned off, a resonance occurs due to the parasitism capacitance of the switches Q12 and Q13 and circuit inductance, so that terminal voltage of the switches Q12 and Q13 becomes zero. A ZVS is realized by turning on the switches Q12 and Q13 at the timing of this terminal voltage.
In FIG. 13, the excitation inductance of the primary winding of the transformer 91 is designed small (the exciting current is large), and the current (exciting current Iex) resulting from the excitation inductance greatly contributes to the ZVS action.    Patent Document 1: Japanese Patent Application Publication No. H07-322613    Patent Document 2: International Patent Application Publication No. WO 2005/025043